Nowadays, in the field of magnetic disk storage apparatuses (hereinafter also referred to as “disk drives”) such as hard disk drives, high recording density and high recording capacity have been achieved by the development of perpendicular magnetic recording and high-frequency-assisted writing.
High-frequency-assisted writing is a recording method of applying a high-frequency magnetic field to a magnetic disk by using a magnetic head including a high-frequency-assisted element. As the high-frequency-assisted element, for example, a spin-torque oscillator (STO) is widely noticed. The disk drive supplies a direct current to the STO through an electrode, thereby drives the STO as oscillation source and generates a high-frequency magnetic field.
In a magnetic disk apparatus using an STO, a direct energizing current is always supplied to the STO in recording data in which a write operation is performed. The write signal input to the magnetic head in recording data includes a relatively low-density region and a relatively high-density region. It has been found that a bit error rate (BER) of data recorded on the disk deteriorates, in write operation in response to a write signal of a low-density region.
When the disk drive supplies a direct current to the STO and records data on the disk, it is inferred that the deterioration in the bit error rate is caused by influence of oscillation of the STO element on a trailing shield disposed on a trailing side of the STO and occurrence of a magnetic field of a direction reverse to that of the main pole in a low-density region. Specifically, it is inferred that the deterioration is caused by increase in the magnetic field of a direction reverse to the magnetic field directly under the main pole, in under the trailing shield and thus disturbance of magnetization of the recording pattern after recording.